1. Field of the Invention
The present invention relates generally to an extrusion processes for extruding a porous substrate, and in one particular implementation to an extrusion process for extruding a porous ceramic substrate.
2. Description of the Related Art
Many processes require rigid substrates for facilitating and supporting various processes. For example, substrates are used in filtering applications to filter particulate matter, separate different substances, or remove bacteria or germs from air. These substrates may be constructed to operate in air, exhaust gases or liquids, and may be manufactured to endure substantial environmental or chemical stresses. In another example, catalytic materials are deposited on the substrate for facilitating chemical reactions. For example, a precious metal may be deposited on an appropriate substrate, and the substrate may then act to catalytically convert dangerous exhaust gases into less noxious gases. Typically, these rigid substrates operate more effectively with a higher porosity.
Porosity is generally defined as the property of a solid material defining the percentage of the total volume of that material which is occupied by open space. For example, a substrate with 50% porosity has half the volume of the substrate occupied by open spaces. In this way, a substrate with a higher porosity has less mass per volume than a substrate with a lower porosity. Some applications benefit from a lower mass substrate. For example, if a substrate is used to support a catalytic process, and the catalytic process operates at an elevated temperature, a substrate with a lower thermal mass will more quickly heat to its operational temperature. In this way, the time for the catalyst to be heated to its operational temperature, i.e., light off time, is reduced by using a more porous and less thermally massive substrate.
Permeability is also an important characteristic for substrates, particularly filtering and catalytic substrates. Permeability is related to porosity, in that permeability is a measure of how easily a fluid, such as a liquid or gas, may flow through the substrate. Most applications benefit from a highly permeable substrate. For example, an internal combustion engine operates more efficiently when the after-treatment filter provides lower back pressure to the engine. Low back pressure is created by using a more highly permeable substrate. Since permeability is more difficult to measure than porosity, porosity is often used as a substitute guide to the permeability of a substrate. However, this is not a particularly accurate characterization, as a substrate may be quite porous but still have limited permeability if the pores are not generally open and interconnected. For example, a Styrofoam drinking cup is formed of a highly porous foam material, but is not permeable to the flow of liquid. Therefore, in considering the importance of porosity and permeability, the pore structure of the substrate must also be examined. In the example of the Styrofoam cup, the Styrofoam material has a closed pore network. This means that the foam contains many non connected and/or closed-ended pores. In this way, there are many voids and open spaces within the foam, but since the pores are not connected, the fluid or gas cannot flow from one side of the foam to the other. As more of the channels begin to interconnect, then fluid paths begin to form from one side to the other. In such a case, the material is said to possess more open pore network. The more connected channels formed through the material, the higher the permeability becomes for the substance. In the case where every pore is connected to at least one other channel, and all pores allow for fluid flow through the entire thickness of the wall formed of the material, the substrate would be defined as having a completely open pore network. It is important to note the difference between cells and pores. Cells refer to the channels that run (generally parallel to each other but not necessarily) through the honeycomb substrate. Often, the honeycomb substrates are referred to in the context of how many cells they have per square inch. For example, a substrate with 200 cells per square inch has 200 channels along the principle axis of the substrate. Pores, on the other hand, refer to the gaps inside the material itself, such as in the material that constitutes the wall separating two parallel channels or cells. Completely or mostly open pore network substrates are not known in the filtering or catalytic industries. Instead, even the most porous available extruded substrates are a hybrid of opened pore and closed pore porosity.
Accordingly, it is highly desirable for many applications that substrates be formed with high porosity, and with an internal pore structure that enables a similarly high permeability. Also, the substrates have to be formed with a sufficiently rigid structure to support the structural and environmental requirements for particular applications. For example, a filter or catalytic converter that is to be attached to internal combustion engine must be able to withstand the likely environmental shock, thermal requirements, and manufacturing and use stresses. Finally, the substrate needs to be produced at a cost low enough to allow for widespread use. For example, in order to affect the level of worldwide pollution from automobiles, a filtering substrate must be affordable and usable in developed as well as developing countries. Accordingly, the overall cost structure to filters and catalytic converter substrates is a substantial consideration in the substrate's design and selected process.
Extrusion has proven to be an efficient and cost-effective process to manufacture rigid substrates of constant cross section. More particularly, extrusion of ceramic powder material is the most widely used process for making filter and catalytic substrates for internal combustion engines. Over the years, the process of extruding powdered ceramics has advanced such that substrates may now be extruded having porosities approaching 60%. These extruded porous substrates have had good strength characteristics, may be flexibly manufactured, may be manufactured at scale, maintain high quality levels, and are very cost-effective. However, extrusion of powdered ceramic material has reached a practical upper limit of porosity, and further increases in porosity appear to result in an unacceptably low strength. For example, as porosity is increased beyond 60%, the extruded ceramic powder substrate has not proven strong enough to operate in the harsh environment of a diesel particulate filter. In another limitation of the known extrusion processes, it has been desired to increase the surface area in a substrate to allow for more efficient catalytic conversion. In order to increase surface area, extruded ceramic powder substrates have tried to increase cell density, but the increase in cell density has resulted in an unacceptable back pressure to the engine. Thus, the extruded ceramic powder substrate does not have sufficient strength at very high porosities, and also produces unacceptable back pressure when there is a need for increased surface area. Accordingly, the extrusion of ceramic powder appears to have reached its practical utility limits.
In an effort to obtain higher porosities, filter suppliers have attempted to move to pleated ceramic papers. Using such pleated ceramic papers, porosities of about 80% are possible with very low back pressure. With such low back pressure, these filters have been used in applications, such as mining, where extremely low back pressure is a necessity. However, the use of the pleated ceramic paper filters has been sporadic, and has not been widely adopted. For example, pleated ceramic papers have not effectively been used in harsh environments. Manufacturing the pleated ceramic papers requires the use of a paper making process that creates ceramic paper structures that are relatively weak, and do not appear to be cost-effective as compared to extruded filters. Telescoping, or unraveling of the pleated paper has been observed in the field, even when the ceramic fibers were coated with tough coatings, such as SiC, using expensive chemical vapor deposition type processes. Further, the formation of pleated ceramic papers allows very little flexibility in cell shape and density. For example, it is difficult to create a paper pleated filter with large inlet channels and smaller outlet channels, which may be desirable in some filtering applications. Accordingly, the use of pleated ceramic papers has not satisfied the requirement for higher porosity filter and catalytic substrates.
In another example of an effort to increase porosity and to avoid the disadvantages of pleated paper, some have built substrates by forming a mass with ceramic precursors and carefully processing the mass to grow mono-crystalline whiskers in a porous pattern. However, growing these crystals in-situ requires careful and accurate control of the curing process, making the process difficult to scale, relatively expensive, and prone to defects. Further, this difficult process only gives a few more percentage points in porosity. Finally, the process only grows a mullite type crystalline whisker, which limits the applicability of the substrate. For example, mullite is known to have a large coefficient of thermal expansion, which makes crystalline mullite whiskers undesirable in many applications needing a wide operational temperature band and sharp temperature transitions.
Accordingly, the industry has a need for a rigid substrate that has high porosity and an associated high permeability. Preferably, the substrate would be formed as a highly desirable open cell network, would be cost-effective to manufacture, and could be manufactured with flexible physical, chemical, and reaction properties.